Volt drop

Module 1 showed that voltage is the push. As current flows round a circuit, every part uses up some of that push. Add up every share and you always get the battery voltage.

• What a volt drop is, and why the load should get almost all the voltage
• How a little extra resistance starves the load
• Why you test with the circuit working
• Where to put the meter leads, and how to read the result

Everything takes a share

In a healthy lamp circuit, the feed side (clamp, wire and connector) takes about 0.1 V, the earth side takes about 0.1 V and the lamp takes the other 11.8 V. That's exactly what we want. The load is where the work gets done, so it should get almost all the voltage. The wiring should take as little as possible.

Each of those shares is a volt drop: the voltage used up across one part of the circuit. You can measure it with a voltmeter by putting one lead on each side of the part.

Now add a problem: a corroded earth. Corrosion adds resistance, say 1 Ω. The lamp still has its 2 Ω, so the total is now 3 Ω. Ohm's law gives 12 ÷ 3 = 4 A, down from 6 A.

That 4 A has to get through the corroded joint. Across 1 Ω, it uses up 4 A × 1 Ω = 4 V. That leaves only 8 V for the lamp, so it glows dim. The battery is fine, the bulb is fine and the fuse is fine. The joint between them is stealing the voltage.

It's the same Ohm's law as Module 1, done in three steps: add up the resistance, find the current, then find the joint's share. The steps under the bench show each one, with the triangle for each.

Where you'll see it

Volt drop causes dim lights, slow wipers and fans, starters that crank slowly, and control units that reset or log strange faults. In each case the parts themselves are fine. It's a connection between them that's using up the voltage.

Here's the trap. Unplug the lamp and measure at its connector, and you'll see the full 12 V. With no current flowing, the corrosion has nothing to hold back, so it doesn't use any voltage. The fault only shows up when the circuit is working.

A volt drop is current × resistance. No current means no drop, however bad the joint is. So test with the circuit switched on and the load working. That's called testing under load.

Why not use an ohmmeter?

An ohmmeter works with the power off and pushes its own tiny test current through the part. That has two problems for this job. A joint that's only just hanging on can read fine with a tiny current, then fail when it has to carry a real load. And the meter's own leads add about 0.1 to 0.3 Ω, which hides the small resistances you're looking for.

A voltmeter across the part, with the circuit working, uses the circuit's real current. At 6 A, even 0.5 Ω shows up as a clear 3 V drop.

Put one meter lead on each side of the part you want to check, such as either end of a wire or across a connection. The meter reads the voltage being lost across just that part.

Check the battery first. Across its terminals (A to H on the bench) it should read close to 12 V, and that's the push you compare everything else against. Then start big and narrow it down. Across the whole feed side (battery positive to the load's feed terminal) tells you if the feed is losing voltage. Across the whole earth side (the load's earth terminal to battery negative) does the same for the earth. Then split the bad side into smaller pieces until one part shows the drop.

A good connection should show almost nothing. A high reading means that part is using voltage it shouldn't, and that's the fault you're looking for. Check the feed side and the earth side, because a bad earth causes just as many problems as a bad feed.

What counts as too much depends on the circuit and how much current it carries. Your manufacturer's data gives the exact limits. Without the data you can still compare: a reading that's many times bigger than the same kind of part elsewhere in the circuit stands out.

One reading tells you nothing at all: any reading taken with the circuit switched off. With no current flowing, even a badly corroded joint reads zero.

A volt drop test is a short routine. Get the order right and it finds the fault in a handful of readings, without unplugging anything. It starts at the battery: if the battery itself is low, every reading round the circuit will be low too, and you could chase a fault that isn't in the wiring. Put these steps in the right order.

The last step matters. A fixed joint should bring the drop right down and the load back to full strength. Testing again under load proves the repair worked.

Each round hides one high-resistance fault somewhere in the circuit. The lamp is on and it's dim. Drag the meter leads onto the test points to find it in as few readings as you can, then say where it is.

Ten questions covering the whole module. You need 8 out of 10 (75%) to pass. Each answer comes with a short explanation.